The present invention pertains generally to walking devices and more particularly to walking devices for large machinery.
Many prior art designs exist for walking devices for large machinery such as disclosed in U.S. Pat. Nos.: 3,901,341, 3,265,145, 3,500,945, 3,114,425, 2,247,782, 2,259,200, 3,249,168, 3,375,892. However, the only known working mechanical devices are the Marion crankpin operated walking device schematically illustrated in FIG. 2, the Bucyrus Erie cam operated walking device schematically illustrated in FIG. 1, and the Page walking device schematically illustrated in FIG. 3. These devices suffer from various disadvantages and limitations.
A primary disadvantage of prior art walking devices relates to travel of the center of gravity of the drag line in a horizontal direction with respect to the lifting force. Each of the prior art devices begins its start of step, i.e., the initial lifting portion of the step, with the vertical lifting force a maximum distance (during its cycle of operation) from the center of gravity of the walking drag line. This causes tub drag weight to be maximized at the start of step position. Since power requirements for lifting the drag line are necessarily maximized at the start of step, and prior to art devices unavoidably impart a horizontal movement, in addition to the vertical lifting movement, an extremely undesirable power curve is generated. Ideally, for a more desirable power curve, the drag line should start the step with low tub drag and end the step with higher tub drag. This would allow the walking device to lift the drag line while imposing minimal resistance imparted by horizontal force vectors, while simultaneously imposing greater resistance to offset reverse loading incurred during the "set down" phase of operation.
Furthermore, maximum tub drag of the start of step causes a smaller percentage of the total weight of the drag line to be placed on the shoes before horizontal movement is initiated, as compared to a device having its vertical lifting force closer to the center of gravity of the drag line and consequently lessens tub drag. Resultant higher horizontal resistance forces generated by horizontal tub drag and lesser total weight placed on the shoes, increases the probability for shoe slippage. To increase weight on the shoes and decrease tub drag, prior art devices have shortened step length which decreases the horizontal distance of the lifting force from the center of gravity since step length varies proportionally with the distance of the lifting force from the center of gravity. This, however, is an unfavorable solution to the problems of an undesirable power curve and shoe slippage since speed of movement is significantly decreased due to shorter step length.
To maximize shoe ground clearance, prior art devices such as shown in FIGS. 1 through 3 use less than half of the available 360 degree shaft rotation to actually lift and cause the drag line to travel in a horizontal direction. Actually, only approximately 140 degrees of the crank shaft rotation is used to lift and move the drag line, resulting in a much less efficient use of the motive power of the device. Moreover, the Marion and Page walking devices schematically illustrated in FIGS. 2 and 3, generate forces on the drag line main structure in a direction opposite to the direction of travel, causing further inefficient power utilization and increased tub drag.
A primary disadvantage of the Bucyrus Erie and Marion walking devices illustrated in FIGS. 1 and 2 is that ground clearance of the structure required to support the crank shaft is minimal, e.g., approximately six feet. Of course, it is desirable to have greater ground clearance to enable bulldozers to operate close to the tub under the main frame when necessary. Since in both the Bucyrus Erie and Marion walking devices, ground clearance is directly related to the length of step, and since the length of step cannot be increased for larger machines due to power considerations, the maximum ground clearance of these devices is limited by their design.
Bearings have also created many problems in prior art larger machinery such as drag lines exceeding 6 million pounds and typically 12 million pounds or more. For example, increased friction and airborne abrasives have necessitated the use of outside bearings in larger machines designed to be used in the western United States. Other than the device described in U.S. Pat. No. 2,259,200, no previous devices were designed to utilize outside bearings. Even through the use of outside bearings and special lubricants, the Bucyrus Erie device illustrated in FIG. 1 is incapable of overcoming the friction problems of the skidding action of the cam within the cam race. The line contact between the cam and cam race, together with skidding action imposed by the cam, creates extremely high friction for the pressures imposed by drag lines exceeding 6 million pounds. Although a later design of the Bucyrus Erie device disclosed in U.S. Pat. No. 3,265,145 overcomes these problems of friction, the modified device has an inherently small step length.
Another disadvantage of prior art devices is that large drag lines require large components which cannot be made by casting or forging or as a stress relieved weldment. Due to the size of these components, they must be made as a fabricated weldment which cannot be stress relieved since they are too large to fit in stress relieving furnaces. For example, the Page model 757 drag line spud measures approximately 37.5 feet high, 12 feet wide, and weighs 89,000 pounds. As a result of the fabrication process, the strength of large prior art components is marginal or insufficient, and failure of these parts in use, especially where ground conditions are less than optimum, is fairly common. In fact, present indications show that the largest excavators for which the Page walking device will operate is 8 million pounds due to the size, strength and weight barrier of fabrication for the spud component.
Shoe ground clearance is also a disadvantage of the Bucyrus Erie and Marion Devices illustrated in FIGS. 1 and 2. Since ground clearance in these devices is a function of cam crankshaft operating radius and consequently step length, in larger devices the step length must be significantly reduced to overcome frictional problems in bearings, undesirable power curves, and shoe slippage. Hence, shoe clearance must be proportionally reduced. The ability to step over uneven ground, impediments such as rocks or stumps, or lift the drag line if it sinks into the ground, while operating in one location for several hours, is therefore reduced.
Similarly, low tub lift height provided by the prior art devices generates greater tub drag and its resultant disadvantages and limitations when used in loose surface conditions prevalent in western surface mining, due to the build-up of several inches or more of ground beneath the dragging side of the tub, consequently increasing tub drag surface area. A higher tub lift would, of course, decrease the area of ground contact.
Another disadvantage of the prior art devices, especially the Bucyrus Erie device illustrated in FIG. 1 and the Page device illustrated in FIG. 3, is that these devices operate by lifting the machine on one radius and moving the machine across the ground by use of a second radius. Use of this twin radius concept places uncentered loads of varying intensity on related walking device components. The result of these eccentricities is a twisting load of great magnitude applied to various components causing frictional problems, and often times, failure of these components.
Another disadvantage of the prior art devices is that the drag line travels across unlevel walking surfaces thereby causing the drag line to slant to one side. This imposes tremendous binding or twisting loads on the cam crank bearing of the Marion device shown in FIG. 2 and large loads on both the cam and cam race of the Bucyrus Erie device shown in FIG. 1. As a result, the Bucyrus Erie and Marion devices have practically no ability to withstand side loading imposed by unlevel walking roads.
Differences between ground elevation and elevation under the shoes causes additional problems with the prior art devices. For example, if the ground elevation under the shoes is less than the ground elevation under the tub, the step length is considerably shortened, and less of the 140.degree. of crank shaft operating radius is used to step the device. As a result, the shoes tend to slip since horizontal movement of the shoes is well underway by the time the shoes contact the ground. Additionally, less tub lifting height under the circumstances causes greater horizontal resistance for the reasons set forth above.
On the other hand, greater elevation under the shoes relative to the ground elevation under the tub causes the stepping length to be extended beyond design parameters. This causes sudden inordinate demand for power at the start of step which causes the prior art devices to overload or stall the walking motors.